The present invention relates generally to methods, apparatus and compositions for making plastic lenses.
It is conventional in the art to produce optical lenses by thermal curing techniques from the polymer of diethylene glycol bis(allyl)-carbonate (DEG-BAC).
The polymer of DEG-BAC exhibits desirable optical and mechanical properties. These properties include high light transmission, high clarity, and high index of refraction together with high abrasion and impact resistance. The properties in the past made DEG-BAC one of the leading monomers in the manufacture of high quality lenses, face shields, sun and safety glasses. Other properties of DEG-BAC, however, such as its slow rate of polymerization, make it an undesirable monomer in the manufacture of these items. Moreover, DEG-BAC, without any additives or co-monomers, produces a hard but somewhat brittle polymer that is very prone to cracking. In addition, DEG-BAC, without additives, tends to adhere tightly to the lens forming molds, often leading to cracking of the molds.
In addition, the thermal curing techniques for polymerizing DEG-BAC to produce optical lenses have several disadvantages and drawbacks. One of the most significant drawbacks is that it may take approximately 12 hours to produce a lens according to thermal curing techniques. A lens forming mold, therefore, can produce at most two lenses per day.
Moreover, thermal curing techniques employ a thermal catalyst so that a polymerizable mixture of DEG-BAC and catalyst will slowly polymerize even while refrigerated. The polymerizable mixture therefore has a very short shelf life and must be used within a short time or it will harden in its container.
Furthermore, the thermal catalysts utilized according to the thermal curing techniques are quite volatile and dangerous to work with, thus requiring extreme care in handling.
Curing of a lens by ultraviolet light presents certain problems that must be overcome to produce a viable lens. Such problems include yellowing of the lens, cracking of the lens or mold, optical distortions in the lens, and premature release of the lens from the mold.
The present invention is directed to methods, apparatus and compositions for making plastic lenses that overcome the disadvantages and drawbacks of the prior art.
The present invention provides methods, apparatus and compositions for making plastic lenses, such as optical lenses for use in eyeglasses and the like.
In one embodiment of the present invention, a method for making plastic lenses is provided in which a polymerizable lens forming material is disposed in a mold cavity defined in part between a first mold member and a second mold member spaced apart from each other by a gasket. Rays of ultraviolet light are directed against either or both of the first and second mold members or the gasket. In a preferred embodiment, the first and second mold members are cooled. In another preferred embodiment, the ultraviolet light is filtered before it impinges on either or both of the first and second mold members.
In another embodiment of the present invention, an apparatus is provided for making plastic lenses which includes a first mold member and a second mold member spaced apart by a gasket, wherein the first and second mold members define a mold cavity. The apparatus includes a generator for generating and directing ultraviolet light against at least one of the first and second mold members. Alternatively, the apparatus includes a generator for generating and directing ultraviolet light against the gasket. The apparatus may also include a means for preventing the transmission of ultraviolet light through the first and second mold members. In addition, the apparatus may include a filter for filtering the ultraviolet light. The filter may be disposed between the generator for generating and directing ultraviolet light and the first mold member, and between the generator for generating and directing ultraviolet light and the second mold member. The apparatus may include a fluid (e.g. air) distributor or a liquid bath for cooling the first and second mold members.
In another embodiment of the invention, the lens forming material may be cooled at relatively low temperatures while being exposed to ultraviolet (xe2x80x9cUVxe2x80x9d) light. The lens forming material may be cooled by directing various flowrates of cooling air towards the mold members. The mold members themselves may be made thinner or thicker to achieve optimum lens curing results. Different lens curvatures may be made from the same mold members by varying the UV intensity patterns on the mold members during cure of the lens forming material. Hardness, cure and rigidity of the lenses made as described above may be improved by demolding the lenses and then subjecting the lenses to high intensity UV light and/or heating.
In still another embodiment of the present invention, a photoinitiator is provided which includes methyl benzoylformate. The photoinitiator may be used with a composition that includes at least one polyethylenic-functional monomer containing two ethylenically unsaturated groups selected from acrylyl and methacrylyl. The photoinitiator may be used with a composition that includes at least one polyethylenic-functional monomer containing three ethylenically unsaturated groups selected from acrylyl and methacrylyl. The compositions may include an aromatic containing bis(allyl carbonate)-functional monomer such as bisphenol A bis(allyl carbonate). The compositions may include 1,6 hexanediol dimethacrylate, trimethylolpropane triacrylate, tetraethylene glycol diacrylate and/or tripropylene glycol diacrylate. In a preferred embodiment, the composition photoinitiator also includes 1-hydroxycyclohexyl phenyl ketone.